In a communications network, it is often desirable to identify and distinguish one transmitter from other transmitters operating within the network. For example, in the radio telephone industry, a cellular telephone system utilizes an electronic serial number (ESN) and a mobile telephone identification number (MIN) to provide a unique identification for each transmitter. When an individual subscriber or other authorized user of a particular cellular telephone wishes to place a phone call, he dials in a telephone number and presses the "Send" button. In response, his cellular telephone transmits its ESN and MIN to the cellular network so the individual subscriber can be charged for the telephone call.
Unfortunately, unscrupulous individuals illegally operate cellular telephones by counterfeiting the ESN and MIN of a valid subscriber's telephone in order to obtain illegal access to the cellular network without paying for the service. The ESN and MIN of a cellular telephone can be obtained by a counterfeiter electronically monitoring the initial transmission of the telephone, and then programming the detected ESN and MIN into another telephone for illegal use. Thus, the mere transmission of the authentic ESN and MIN is by itself inadequate to protect a cellular telephone system from misuse by counterfeiters. When a cell phone initiates a call, it transmits its ESN and MIN as an identification. While the cell phone is identified by its ESN and MIN, it cannot be considered as an authorized cell phone because it is not known whether the ESN and MIN have been transmitted by the authorized cell phone or a fraudulent cell phone. For purposes of the present description a cell phone identified on the basis of the transmitted ESN and MIN is designated as an unauthenticated cell phone until it is determined to be an authorized cell phone or a fraudulent cell phone.
In an effort to provide additional security, some cellular systems and other wireless services, authenticate mobile units based on the radio frequency (RF) transmission of data by the mobile unit during a call set-up process. Rather than identify the mobile unit by its ESN and MIN alone, the system identifies a cellular telephone by its transmission characteristics. In this manner, the cellular system operator can reject calls from illegitimate cellular telephones even when those cellular telephones transmit valid ESN and MIN numbers. For example, in U.S. Pat. No. 5,005,210 issued to Ferrell on Apr. 2, 1991 ("the Ferrell patent"), a system is described that analyzes certain transmitter characteristics in an effort to identify the transmitter type. The system in the Ferrell patent analyzes the manner in which the modulator makes a transition to the designated carrier frequency. This transient response is used to identify the type of transmitter.
While the Ferrell patent describes one class of transmission characteristics that can be used as a fingerprint, other transmission characteristics are also known in the art. For example, U.S. Pat. No. 5,420,910 issued to Rudokas et al. on May 30, 1995 ("the Rudokas patent"), describes an identifier, such as a radio frequency signature, that can be used to positively identify a valid cellular telephone or a known fraudulent telephone. Other types of signature authentication systems are also known in the art and need not be described herein. These transmission characteristics, from whatever source they are derived, can be processed in different manners to create a "fingerprint" of the individual transmitter. The analogy with fingerprints is used because each transmitter fingerprint is believed to be completely unique. The transmitter fingerprint can be used to determine whether the transmission characteristics of the unauthenticated transmitter match the stored fingerprint of the authorized transmitter corresponding to the transmitted ESN and MIN. In such manner, the fingerprint is used with cellular telephone calls to authenticate the cellular telephone.
Fingerprint authentication systems all require at least one transmission characteristic waveform, known to be generated by the authentic cellular telephone, to be used as a reference waveform for the fingerprint authentication system. Some systems may rely more than one reference waveforms to generate the fingerprint. One drawback of these fingerprint authentication systems is that a substantial amount of data processing time may be required to establish the fingerprint. However, this data processing is not time dependent and may be performed over a period of time.
In contrast, the process of comparing the transmission characteristic of the unauthenticated transmitter with the stored fingerprint must be performed in real-time to effectively deny or terminate fraudulent calls. Some existing cellular telephone systems use analysis techniques that are not real time. If a call is subsequently determined to be fraudulent, the destination telephone number is added to a list of known fraudulent numbers. The cellular telephone system checks the list of known fraudulent numbers to prevent any subsequent telephone calls from being placed to a known fraudulent number.
Real-time analysis techniques are subject to possible attack by fraudulent users who attempt to overload the capabilities of a cellular telephone system and force the acceptance of fraudulent calls. The theory is that the cellular telephone system will be forced to allow calls to be placed without the benefit of real-time fingerprint authentication if the system can be so overloaded that it cannot adequately keep up with the call load. Rather than force customers to wait while calls are authenticated, the overloaded system will have to allow calls to be placed without the fingerprint analysis.
Therefore, it can be appreciated that there is a significant need for a system and method for detection of redial fraud and preventing overloading of a cellular telephone system's processing capabilities. The present invention provides this and other advantages as will be illustrated by the following description and accompanying figures.